Over recent years, with advance of higher performance and further downsizing of imaging elements using solid-state imaging elements such as CCD (Charge Coupled Device)-image sensors or CMOS (Complementary Metal Oxide Semiconductor) image sensors, mobile phones and portable information terminals provided with an image pickup device using such an imaging element are prevailing. Further, in imaging lenses (imaging optical systems) installed in these image pickup devices, there is an increasing demand for further downsizing and higher performances. For an imaging lens for such usage, a five-lens optical system has been recently proposed, in addition to a three-lens or four-lens optical system.
In general, a highly-performance image pickup device is mounted with a function referred to as so-called auto focusing. However, since in the conventional configuration, whole three to five lenses are extended altogether, the following problems have become obvious: the size of a drive device is increased, whereby the size of the entire lens unit cannot be reduced; dust generated in a portion having a drive section causes an adverse effect on image quality, and an eccentric error of the drive device decreased the image quality. Basically, since there is a trade-off between performance enhancement and size reduction, these issues are technically difficult to be compatible. Therefore, to overcome this problem, it is necessary to conduct a fundamental review of the focusing system base on the conventional whole lens focusing method.
As one example of such efforts, Patent Document 1 and Patent Document 2 are cited. In the optical system disclosed in Patent Document 1, mainly by extending only the first lens, the size of a drive device is reduced. Further, as another example, in the optical system disclosed in Patent Document 2, of four lenses constituting a lens, only the second lens is extended to focus.